Fabrication of binary metal–organic frameworks of Ni–Mn@ZIFs(Cox·Zn1−xO) decorated on CF/CuO nanowire for high-performance electrochemical pseudocapacitors

Herein, metal–organic frameworks (MOFs) derived nanoflower-like based binary transition metal (Ni–Mn) are successfully fabricated by a simple synthesis method. The fabricated nanoflower-like structure displays a unique nanoflower-like architecture and internal porous channels constructed by MOF coated on CuO/CF/ZIFs (Cox·Zn1−xO) substrate, which is beneficial for the penetration of electrolyte and electron/ion transportation. The as-prepared CF/CuO/ZIFs (Cox·Zn1−xO)@BMOF(Ni–Mn) electrode materials present significant synergy among transition metal ions, contributing to enhanced electrochemical performances. The as-prepared CF/CuO/ZIFs (Cox·Zn1−xO)@BMOF(Ni–Mn) hybrid nanoflower-like display a high specific capacity of 1249.99 C g−1 at 1 A g−1 and the specific capacitance retention is about 91.74% after 5000 cycles. In addition, the as-assembled CF/CuO/ZIFs (Cox·Zn1−xO)@BMOF(Ni–Mn)//AC asymmetric supercapacitor (ASC) device exhibited a maximum energy density of 21.77 Wh·kg−1 at a power density of 799 W kg−1, and the capacity retention rate after 5000 charge and discharge cycles was 88.52%.


Characterization
Porosity and high surface area are two fundamental factors that extensively determine the effectiveness and efficiency of nanostructures in energy storage applications.The as-prepared electrodes were examined with FESEM analysis to investigate the morphology, sizes, and distribution of grains.As can be seen in Fig. 1A-J, introducing elemental components can induce morphological alterations in an electrode surface, which can either augment or reduce its characteristics.The FESEM of commercial CF exhibits a smooth surface devoid of any pores, as expected (shown in Fig. 1A,B).The study presents Cu (OH) 2 and CuO fabricated on CF substrates that were synthesized using different approaches.As shown in Fig. 1C,D, the high-magnification FESEM image of Cu (OH) 2 confirms the structure of nanowires, characterized by a substantial length-to-diameter ratio.Under the calcination step, the CuO morphology displays the formation of multi-branched nanowires and nanorodtype structures (Fig. 1E,F).The porous surface of the CuO layer allows for an open structure in supercapacitor electrodes, which promotes fast ion diffusion at the interface between the electrolyte and the electrode.The incorporation of ZIFs (Co x •Zn 1−x O) into the CuO substrate changed its morphology.As shown in Fig. 1G,H.The addition of ZIFs (Co x •Zn 1−x O) increased the agglomeration of spherical morphology, resulting in the formation of a uniformly structured material with high porosity.The high surface area of CF/CuO/ZIFs (Co x •Zn 1−x O) also offers a greater number of electro-active sites, as well as a wider contact area between the electrolyte and electrode.This results in a higher charge-discharge capacity of supercapacitors.The high-resolution image of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) exhibited a nanoflower-like structure, Fig. 1I,J.Among the electrodes with different structures, nanoflower-like structures of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) possess numerous advantages among the other electrode morphologies.Due to their shorter paths for ion and electron transport, high surface area, and structural stress reduction across consecutive electrochemical cycles.The result related to the morphology of electrodes and active sites has been deeply investigated by different electrochemical methods.
The elemental compositions of the electrode materials were analyzed using energy dispersive spectroscopy (EDS).Figure 2A-E  The chemical states and elemental compositions of the CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode were determined through the utilization of X-ray Photoelectron Spectroscopy (XPS).For the XPS analysis, the binding energy of the C 1s peak calibrated at 284.5 eV was utilized as a charge reference.Figure 4A displays the complete survey scan spectra of the CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode, revealing the presence of key components including O, Mn, Ni, Zn, Co, and Cu.Deconvoluting the O 1s yielded three probable Gaussian fits.As can be seen in Fig. 4B, the peak recorded at 529.3 eV corresponded to metal-oxygen bonding, the peak referred to the lattice oxygen was observed at 533.1 eV, the peak positioned at 534.4 assigned to the presence of adsorbed oxygen 39 .In the high-resolution Mn 2p spectrum of the CF/CuO/ZIFs (Co x •Zn 1−x O)@ BMOF(Ni-Mn) electrode, the doublet peaks corresponding to Mn 2p 3/2 and Mn 2p 1/2 were positioned at 643.62 and 654.15 eV, respectively (shown in Fig. 4C).Three peaks were observed at 641.8, 644.90, and 648.3 eV in the deconvoluted Mn 2p 3/2 spectrum indicating that the Mn 2+ and Mn 3+ (oxidation states of Mn in binary MOF structure) and satellite peak, respectively.The deconvoluted Mn 2p 1/2 peak displays the presence of a primary peak located at 654.15 eV corresponding to Mn 3+40,41 .Figure 4D exhibits the XPS measurement of Ni 2p spectrum for the electrode.The lower peak intensity seen at 854.6 eV refers to Ni 2p 3/2 .The aforementioned characteristics are fundamentally corresponded to the value of the co-existence of Ni 2+ and Ni 3+ .Another peak position in Ni 2p 3/2 observed at 859.7 refers to the satellite peak.The deconvoluted Ni 2p exhibits two peaks at high peak intensity, with centers at approximately 875.2 and 877.90 eV, which are attributed to the Ni 2p 1/2 and satellite peaks, respectively.Mn and Ni both have valence states of + 2 and + 3 at the same time 42,43 .The presence of different valence states of metal elements will enhance the catalytic activity active site and strengthen the redox performance of the MOF, hence providing considerable benefits to the energy storage performance.The presence of Ni and Mn embedded in the MOF structure during the final synthesis step is verified in Fig. 4C,D.As can be seen in Fig. 4E, the Co 2p spectra represent two peaks in the curve at 782.6 and 797.5 eV attributed to Co 2p 3/2 and Co 2p 1/2 states, respectively.The Co 2p 3/2 spectrum was deconvoluted into two fitted peaks at 781.20 eV and 783.41 eV,   44 .As shown in Fig. 4F, the high-resolution Zn 2p spectra exhibits two distinct peaks at around 1022.4 and 1044.1 eV, which correspond to the Zn 2p 3/2 and Zn 2p 1/2 oxidation state, respectively.The presence of these peaks can be ascribed to the Zn 2+ oxidation state.Therefore, it can be inferred that the doped ZnO polycrystal contains the Co phase.Figure 4G displays the Cu 2p spectrum with a prominent level of detail.The two primary peaks, located at around 935.4 eV and 948.2 eV, correspond to the Cu 2p 3/2 and Cu 2p 1/2 peaks, respectively.The Cu 2p 3/2 and Cu 2p 1/2 states are further analyzed, revealing the existence of both 1+ and 2+ oxidation states 45,46 .The conspicuous satellite peak suggests that the predominant oxidation state of copper was Cu 2+ .The inclusion of metallics in the final electrode serves as active sites for enhanced Faradaic redox reactions, resulting in superior capacitive performance.This finding provides more evidence that binary MOF doping can increase the active site in CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn), which aligns well with the observations made through the FESEM image.

Electrochemical performance
To assess the efficacy of CF/Cu(OH) 2 , CF/CuO, CF/CuO/ZIFs (Co x •Zn 1−x O), and CF/CuO/ZIFs (Co x •Zn 1−x O)@ BMOF(Ni-Mn) electrode as energy storage materials, we conducted investigations using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques.The studies were conducted utilizing a 3-electrode setup in an aqueous electrolyte containing 3.0 M KOH.As can be seen in Fig. 5I, the fabricated electrode materials were evaluated by CV analysis at a scan rate of 20mV/s.The purpose was to examine the potential pseudo-faradaic contributions throughout the potential range of 0.0-6.0V vs. Hg/HgO at room temperature.Nevertheless, a quasi-reversible peak was detected at approximately 0.46 V vs. Hg/HgO for CF/Cu(OH) 2 , indicating the oxidation of copper species.Additionally, a reduction peak was identified at around 0.27 V vs. Hg/HgO during the negative scan.Typical CV curves reflecting the nature of the electrochemical behavior of CF/CuO in a potential range of 0.0 to 0.6 V vs. Hg/HgO are shown.The CF/CuO/ ZIFs (Co x •Zn 1−x O) electrodes showed a board redox peak, which confirms their faradaic behavior that could correspond to the reversible redox of Zn and Co, with an additional redox peak of Cu 1+ /Cu 2+ for CuO substrate.These results illustrate the ratio/proportion of the integral area, suggesting that the combination CF/CuO/ZIFs (Co x •Zn 1−x O) has a greater integrated area compared to the other electrode materials.Furthermore, the addition of binary metal MOF in the structure resulted in a substantial increase in the area under the CV curve.This suggests that the inclusion of Mn and Ni into the MOF structure enhances the capacitive performance of the electrodes.The entire reaction can be attributed to the combination of M (representing Cu/Zn/Co/Ni/Mn) and OH anion derived from the KOH electrolyte.
Among all CVs records best specific capacity related to CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) due to its high surface area and good crystallinity as it was confirmed via FE-SEM images.
As shown in Fig. 5II, the CV curves of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) are also studied at various scan rates.As the scan rate increased from 5 to 100 mV s −1 , the current density area of the redox peak became larger.The almost symmetrical CV curves for CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) suggest that the electrode materials were advantageous for fast redox reactions, and exceptional rate capability.Furthermore, the anodic and cathodic peaks exhibited a shift towards positive and negative potentials, respectively.This shift can be attributed to the polarization of the electrode materials as well as the fast transport of electrons and ions.On the other hand, the scan rate increase results in decreased electrolyte interaction with the electrochemically active species, which consequently hinders reaction kinetics.
Furthermore, the analysis of both Faradaic and non-Faradaic processes can be conducted by plotting the logarithm of the peak current (anodic and cathodic) against the logarithm of the scan rate (Log I = b log(ʋ) + log(a)).The scan rate is indicated by ʋ, current (i), whereas a and b are constants.The value of b can be ascertained from the slope of the aforementioned plot.On the other hand, the charge storage characteristics of the electrode materials are determined by the value of b.When the value of b is 1, the process is solely capacitive, namely an electric double-layer capacitor (EDLC).Conversely, when the value of b is 0.5, the process can be categorized as a diffusion-controlled faradaic reaction.The b values were determined for CF/CuO/ZIFs (Co x •Zn 1−x O)@ BMOF(Ni-Mn), yielding a cathodic peak value of 0.512 and an anodic peak value of − 0.503 (As shown in Fig. 5III,IV.These findings indicate that the electrode's charge storage mechanism was mostly influenced by a diffusion-controlled process.As a result, the electrode displayed a behavior like that of a battery type.This study

Equation Deform equation
The CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) exhibits a leading contribution from diffusion control over kinetic control, suggesting that the response is mostly controlled by diffusion.
As can be seen in Fig. 6I, the CF/Cu(OH) 2 and CF/CuO electrodes show a limited capacity to maintain their performance at different current densities, with a rate capability of 10.12 and 12.23 respectively, primarily due to the absence of transition metals in electrode materials.Comparatively, the CF/CuO/ZIFs(Co x •Zn 1−x O) and CF/CuO/ZIFs(Co x •Zn 1−x O)@BMOF(Ni-Mn) both show the enhanced rate capabilities of 35.83% and 64.05% respectively.This result can be ascribed to the CF/CuO/ZIFs(Cox•Zn 1−x O) substrate synergetic effect and the electroactivity (Ni/Mn) in the binary MOF and the rate capabilities of the electrode material were increased due to the integration of Ni/Mn to form nanoflower-like MOF as shown in FESEM images.
To assess the conductivity and rate of charge transfer of the fabricated electrodes, Electrochemical Impedance Spectroscopy (EIS) was performed.The frequency range used was 0.1-104 kHz (shown in Fig. 6II).The data obtained using EIS is presented in the form of a Nyquist plot.The resistance is attributed to three factors: the electrolyte's resistance, the active material's resistance in the electrode, and the resistance at the interface between the current collector and the active material.Each data point on the Nyquist plot represents an impedance at a distinct frequency.The analogous series resistance (Rs) or solution resistance is the point where the real axis intersects in the high-frequency zone.On the other hand, a semicircle observed at high frequencies signifies a direct relationship between the charge transfer resistance (Rct) and the diameter of the semicircle.The values of Rct and Rs for each electrode material are provided in Table 1.It can be contended that the resistance values (Rs) stem from a shared point and stay uniform across all electrodes.
It can be concluded that the addition of Ni-Mn-based MOF to CF/CuO/ZIFs (Co x •Zn 1−x O) led to a significant decrease in its resistance, due to the improvement in its conductivity, hence strengthening its electrochemical performance.These EIS findings align well with the results obtained from GCD and CV tests.
Figure 6III demonstrates the Stability test of the CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode, which underwent 5000 GCD cycles at a current density of 10 A g −1 .The Ni-Mn-based MOF introduced to CF/ CuO/ZIFs (Co x •Zn 1−x O) was well maintained and preserved even after 5000 cycles.In addition, the electrode materials exhibited exceptional reversibility, as evidenced by the attainment of a 98.7% coulombic efficiency after 5000 cycles.The cycling performance of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) was evaluated, revealing a cycling stability of 91.74% after 5000 cycles.Hence, it can be inferred that the addition of Ni-Mn-based MOF is the optimal choice for augmenting the electrochemical reactivity of CF/CuO/ ZIFs (Co x •Zn 1−x O), making it a distinctive alternative among electrode materials for supercapacitors.

Asymmetric supercapacitor (ASC)
CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode was used as the positive electrode for the asymmetric supercapacitor (ASC) device because it performed so well as a supercapacitor.The energy density of the ASC device can be increased by combining electrode materials with different chemical properties.To achieve this as a high-capacity supercapacitor, CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) material can be combined with the EDLC-type material (such as activated carbon (AC)).As can be seen in Fig. 7I, the CV plots of the CF/CuO/ ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode (ranging from 0.0 to 0.6 V) and AC electrode (ranging from − 1.0 to 0.0 V) materials indicate that the high-capacity supercapacitor device can function across a broad potential range of 1.6 V.The CV plots of the negative electrode, consisting of nickel foam coated with AC, display a nearly rectangular shape within the stable potential range, indicating the characteristic behavior of EDLC-type material.The CV profile of the positive electrode (CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)) exhibits two separate redox peaks, which is indicative of the Faradaic-pseudo capacitance behavior.To assess the stable potential window of the as-assembled cell, a set of CV curves was obtained, as depicted in Fig. 7II.These curves reveal a consistent voltage range of 0.0 to 1.6 V, indicating stability.Expanding the potential window further will cause phenomena such as oxygen evolution reaction, water splitting, and electrolyte decomposition, resulting in a reduction in capacity and disruption of the system's performance.The remarkable performance of the fabricated device, which is attributed to the combined effect of the EDLC-type material (AC) and the faradaic response  of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) can be observed in Fig. 7IV.This CV profile was recorded at a various scan rate (10 to 100 mV s −1 ) for CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)//AC device exhibit high performance capacitive behavior.The CV profile maintained its shape despite the higher scan rates, suggesting that the ACS device has a good-rate capability, excellent reversibility, fast charge/discharge properties, and rapid electron transfer kinetics.
As shown in Fig. 7III, the GCD curves of the ACS device were recorded at current densities of 1 A g −1 in the potential window range of 0.0 to 1.6 V.The GCD curves closely align with the CV curves, and the observed asymmetrical GCD curve for CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)//AC indicates battery-like behavior.The specific capacity of the as-fabricated ACS device was estimated by evaluating the GCD curves recorded at various current densities (1 to 10 A g −1 ) that have similar shapes that indicate the possibility of electrochemical reversibility (shown in Fig. 7V).The ACS device yielded a maximum specific capacity of 100.47 C g −1 at a current density of 1 A g −1 .Figure 7VI displays the specific capacity values achieved at various current densities.The specific capacity of CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)//AC device at 1.0, 2.0, 3.0, 5.0, 7.0, 8.0, 9.0, and 10 A g −1 are 100.47,98.14, 97.48, 94.99, 92.89, 88.6, 72.24 and 72.17 C g −1 , respectively.Hence, the specific capacity retains 72.2% of its initial value when the current density is raised to 10.0 A g −1 .The durability and coulombic efficiency of the ACS device in its developed state is also a crucial component in evaluating the overall performance of practical applications.The proposed energy storage mechanism is depicted in Fig. 8.The CF/ CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode facilitates the oxidation reaction in the KOH electrolyte by using OH − ions during the charging phase.During discharge, there is an electrochemical reduction.
As shown in Fig. 9I, the ACS device underwent 5000 charge-discharge cycles at a discharge rate of 10 A g −1 .The cell maintained 88.5% of its initial specific capacity, demonstrating the strong and durable quality of the electrode material used.The first and final cycles are displayed in the inset of Fig. 9I.On the other hand, the ACS device had a columbic efficiency of 88.52% after 5000 cycles of charge-discharge.Energy density and power density are crucial factors in assessing the performance of ASC devices.The experimental findings indicate that the CF/CuO@Zn/Co-MOF(Ni-Mn)//AC device possesses an energy density of 21.77 Wh kg −1 and a power density of 799.19 Wh kg −1 .
Notably, the energy density and power density of the ASC device were compared to those of some of the previous devices based on the MOF and multi-metallic electrode with the same transition metal (shown in Fig. 9II).Figure 9III demonstrates the effective application of the ASC device, where two ACS devices were connected in series, to power blue light-emitting diodes (LEDs).This demonstrates the practical use of CF/CuO/ ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) electrode materials.Finally, an in-depth analysis of the AC asymmetric supercapacitor was conducted in comparison to other electrochemical pseudo capacitors (shown in Table 2). .The Cyclic performance and coulombic efficiency were 91.74% and 98.7%, respectively after 5000 GCD cycles.The assembled asymmetric device displays a retention in capacity of 88.52% after 5000 GCD cycles and delivered a high energy density of 21.77 Wh kg −1 and a power density of 799.19 Wh kg −1 .The noteworthy results indicated that combining and designing binary MOF and transition metal oxide on a CuO substrate is a potential approach to enhance the efficiency of electrodes in energy storage applications.

Chemicals and apparatus
The chemical materials utilized in this work were employed without additional purification.Analytical grade potassium hydroxide (KOH), ethanol (C 2 H 6 O), and hexamethylenetetramine ((CH 2 ) 6 N 4 ), activated carbon  www.nature.com/scientificreports/

Assembly of asymmetrical supercapacitors
Assembly of the ASC devices involved the utilization of activated carbon (AC) as the negative electrode, and the CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn) (prepared as such in a three-electrode system) as the positive electrode.The asymmetric supercapacitor device utilizes a Whatman paper to prevent a short circuit between the positive and negative electrodes.To optimize the performance and ensure a broad potential window in the battery-like SCs system, the charge must be in equilibrium between the positive (q + ) and negative (q − ) electrodes, with the ideal mass ratio (m + /m − ) calculated using the subsequent equation: according to the preceding equation, the ideal mass ratio for the as-fabricated asymmetric supercapacitor is almost 0.157.

Figure 9 .
Figure 9. (I) Cyclic stability and coulombic efficiency test for 5000 cycles of charge/discharge.(II) The Ragone plot for energy and powder densities of the CF/CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)//AC device along with a comparison of previously reported similar works.(III) The image of the LED being powered by the CF/ CuO/ZIFs (Co x •Zn 1−x O)@BMOF(Ni-Mn)//AC device.

The oxidation states Co 2p 3/2 and Co 2p 1/2 are confirmed by the presence of two satellite peaks at 785.3 eV and 805.71 eV, respectively
which can be attributed to the Co 2+ and Co 3+ oxidation states, respectively.On the other hand, The Co 2p 1/2 was deconvoluted into two fitted peaks at 796.41 eV and 797.61 eV, indicating the presence of Co 2+ and Co 3+ oxidation states, respectively.

Table 1 .
Solution and charge transfer resistance (Rs and Rct) of electrode materials.